Crystal-growing apparatus and methods



March 13, 1962 w. F. LEVERTON CRYSTAL-GROWING APPARATUS AND METHODS Original Filed July s, 1953 /A/l/ENTOR WAL TER LEI/ER'ON atent 3,25,l9i Patented Mar. 13, 1962 now Patent No. 2,944,875, dated July 12, 1960. Divided and this application Feb. lt), 1969, Ser. No.

7 Claims. (Cl. 148--1.6)

This invention relates to an apparatus and a method used to grow single crystals of uniform electrical resistivity to be employed in the manufacture of transistors and crystal rectifiers, and specifically to the crucible used to obtain such crystals from a melt of semiconductor ma terial.

This application is a division of my copending application, Serial No. 361480; filed July 13, 1953, now Patent No. 2,944,875.

In the preparation of semiconductor single crystals, the seed-pulling method has been used with consderable success. However, in order that these single crystals may be useful for rectifiers and transistors, the electrical resistivity of the crystal should be uniform throughout its length and the crystal should `be readily reproducible. Since several important parameters of transistors vary more or less linearly with the resistivity of the single crystal, the diflculty of making units with predictable characteristics is greatly increased if uniform resistivity is not obtained in the crystal during the growing process.

The change in resistivity along the length of a single crystal may be attributed primarily to the fact that the p-type and n-type doping ,agents, such as arsenic, antimony, gallium and indium, commonly added to a melt of pure semiconductor material, such as germanium or silicon, are more soluble in the liquid semiconductor material than in the solid semiconductor material. Hence, in a growing crystal the concentration of doping agent in the solid semiconductor crystal, hereinafiter referred to as C is less than the concentration of doping agent in the adjacent liquid semiconductor material, hereinafter referred to as C Therefore, as a crystal is progressively grown by the seed-pulling method, a steadily ncreasing concentration of doping agent is left in the remaining melt of semiconductor material.

For example, the value of of liquid semiconductor material remaining in the crucible. Since s CL remains efiectively constant throughout the crystal-growing process, an increased C leads to an ncreasing C and since the electrical resistivity of the crystal is inversely proportonal to C the resistivity progressively decreases throughout the length of the crystal. For example, in growing a 400-gram crystal of doped germanium from a SDO-gram melt, the resistivity may decrease by a factor of four or five along the length of the crystal. Such variations necessitate detailed selection and classifi cation of the slices made from the single crystal material, and this is both wasteful of time and material. Whereas several methods have been proposed to improve this situaeach is provided with a cylindrieal recess theren adapted tion, such as precisely varying the crystal-pulling rate, using larger melts of semiconductor material or controlling the rate of solidification, these methods involve complicated procedures and equipment and the chances of growing an impure or imperfect crystal are increased.

This invention relates to means whereby the concentration of doping agent in the liqud semiconductor material, C and the Volume of the semiconductor material is kept substantially constant throughout the seed-pulling process so that a single crystal of uniforrn resistivity may be grown. In one embodiment of the invention a pair of cylindrical crucibles, the second of which is designed to fit loosely into the first, is used. The second or inner crucible is provided with a small hole drilled through the bottom thereof. Thus, the crucibles are proportioned so that, when a charge of high purity undoped or lightly doped semiconductor material is melted in the outer crucible, the inner crucible may be placed therein so the bottom thereof rests on the surface of the melt. By pushing down slightly on the inner crucible the melt may be forced through the hole until the inner crucible assumes a position of equilibrium so that it is floating in the melt and contains a portion of the melt therein. A doping agent is then added to the melt in the inner crucible. Thus, the outer crucible contains a melt of lightly doped or undoped semiconductor material and the inner crucible holds a melt of semiconductor material having a substantially greater concentration of doping agent therein.

A single crystal may be grown from the melt described above by the seed-pulling method, and as the crystal grows, the lightly doped semiconductor material Will flow from the outer crucible into the inner crucible to maintain the equilibrium level. Therefore, until the inner crucible touches the bottom of the outer crucible, the Volume of liquid semiconductor material in the inner crucible will remain exactly constant. Since only a small fraction of the doping agent is used up in the growing crystal, C remains practically constant throughout the process and the resistivity of the crystal is uniform. The small decrease in C during growth due to the fraction of doping agent used up by the crystal is corrected by lightly doping the entire original melt, as previously mentioned, with a quantity of doping agent equivalent to that used up by the crystal.

With the easily fabricated apparatus described above, large p-type or n-type single crystals having uniform electrical resistivity throughout may be grown. This novel device is easy to operate and the furnace structure used to heat the crucibles and the operation thereof is greatly simplified because the melt is kept at a fixed temperature throughout the growng process. Also, no elaborate equipment for programming the pulling speed is required.

,This invention and the features thereof will be understood more clearly and fully from the following detailed description of one embodiment of the invention with reference to the accompanying drawing whereiu a schematic view of the crystal-growing apparatus is shown.

Referrng now to the drawing, a pair of crucibles 1 and 2 made in accordance with this particular embodiment of the invention is shown mounted within a crystalgrowing apparatus. For purposes of illustration, it will be assumed thatthese crucibles are to be inductively heated and that a germanium single crystal is to be grown. Therefore, the crucibles 1 and 2 should be electrcally conductive, thermally responsive to the heating means, chemcally inert with respect to germanium and readily heated in a high-frequency field. Crucibles made of high purity graphte fulfill these requirements and have been used successfully for the purposes of this invention. Both of the crucibles shown are cylindrical in shape and to hold a melt of semiconductor material, in this instance germanium. The inner crucible 2 is designed to fit into the recess in the outer crucible 1 and should be free to move up or down therein. The inner crucible 2 is provided with a small opening 3 extending through the floor thereof. This opening should be so proportioned that, when a crystal of germanium is drawn, as explained below, the diifusion or mixing of the doping agent through the opening 3 to the outer crucible 1 will be undetectable. For example, in a crucible approximately twice the size of the inner crucible 2 shown in the drawing, an opening one-eighth inch in diameter and one-fourth inch long has been used successfully. It should be noted that the size of the crucibles also may be varied according to the density, amount and type of semiconductor material to be melted therein.

The outer crucible 1 may be mounted and supported on a frame 4 which is, in turn, fixed with a flange 5 to the floor of a cylindrical furnace chamber 6. The walls of the chamber 6 may be made of quartz, for example, and are designed to enclose the crystal-growing apparatus. A series of high-frequency induction coils 7 may be disposed around the outside of the chamber 6 in the area adjacent to the crucibles to heat these crucibles sufiiciently to melt a charge of germanium to be placed therein. These coils are also used to maintain melt at a predetermined temperature during the crystal-growing process.

The operation of the crystal-growing apparatus may be nitiated by first placing a charge of lightly doped germanium in the outer crucible 1 and heating it to about 950 degrees centigrade or slightly above the melting point of germanium. The inner crucible 2 is then lowered into the recess of the outer crucible 1 and is pushed down against the melt therein so that germanium begins to flow through the opening 3 and into the inner crucible 2. The fiow of germanium is continued until the inner crucible has assumed a position of equilibriurn. At this stage of the process the inner crucible should be floating in a germanium melt and should contain a portion of the melt therein, as shown in the drawing. A pellet of doping material, such as a predetermined amount of antimony it` an n-type crystal is to be grown, may then be placed in the melt in the inner crucible 2. It should be noted that the doping agent should be the same as that used to slightly dope the entire original melt. Thus, the inner crucible will contain a melt of germanium 8 having a predetermined concentration of doping agent, which has been selected in accordance with the characteristics desired in the finished single crystal, and the outer crucible 1 will contain a slightly doped melt of germanium 9. The amount of doping agent to be used in the original melt is determined by evaluating the quantity of doping material that will be used up by the single crystal during the growing process. For example, in a typical crystalgrowing operation two to three percent of the doping agent is removed from the melt by the crystal. Therefore, the original melt should contain an equivalent amount of doping agent, that is, two to three percent of the amount of doping material used in the pellet. As explained below, this amount will exactly replace the quantity of doping material used up by the crystal.

The actual crystal-growing operation may be commenced by lowering a seed crystal, not shown, which is fixed to a pull rod 10 by a clamp 11, into the melt 8 in the inner crucible 2. The pull rod is then rotated and slowly raised so that a crystal 12 may be started and grown. Thus, as the crystal is slowly grown, the melt 9 in the outer crucible 1 will flow through the opening '3 into the inner crucible 2 to maintain the equilibrium level. Since the Volume of the germanium melt 8 in the inner crucible is constant during the growing process and the fraction of doping agent used up in growing the crystal is replaced by the doping agent in the lower melt, C remains constant and the electrical resistivity throughout the crystal remains uniform. The process is completed when the lower melt 9 has been exhausted and the inner crucible 2 rests on the floor of the outer crucible 1. It should be noted that, from the time the initial charge is melted until the crystal-growing process is completed, the chamber 6 is continuously flushed with a protective gas, such as argon, for example, to prevent atmospheric contaminants and undesirable reactions from affecting the crystal. Therefore, the chamber 6 is provided with an intake tube 13 and an exhaust tube 14 whereby the protective gas may enter and leave the chamber.

However, it should be understood that this invention is not limited to the particular details described above, as many equivalents will suggest themselves to those skilled in the art. For example, the doping agents and semiconductor materials used and the relative concentrations thereof may be varied as desired. Furthermore, the melt in the outer crucible 1 may be a pure undoped semiconductor material because the Volume of the melt in the inner crucible 2 is kept substantially constant during the crystal-growing process, and under these conditions the resistivity of a slightly doped single crystal is negligibly afiected by the small decrease in concentration of doping agent in the melt. Likewise, the crucibles 1 and 2 can be resistance heated as Well as inductively heated and may be made of materials other than graphite depending upon the chemical and physical properties of the semiconductor material to be melted therein. Therefore, it is desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. The method of growing a single crystal of uniform resistivity, said method comprising pulling a single crystal from an initial melt of semiconductive material containing a doping agent, causing a portion of a second melt of semiconductive material to be added to said initial melt at a rate dependent upon the rate at which semiconductive material is removed from said initial melt by said crystal-pulling step whereby the Volume of said initial melt is maintained substantially constant during the pulling operation, and adding a concentration of doping agent to said initial melt substantially equal to the concentration of doping agent removed by said growing crystal.

2. The method of growing a single crystal of uniform resistivity, said method comprising heating and maintaining an initial melt of semiconductor material containing a doping agent and a second melt of said material at substantially the same temperature, pulling a single crystal from said initial melt of said material, causing a portion of a second melt of semiconductive material to be added to said initial melt at a rate dependent upon the rate at which semiconductive material is removed from said initial melt by said crystal-pulling step whereby the Volume of said initial melt is maintained substantially constant during said pulling operation, and maintaining the concentration of doping agent in said initial melt substantially constant during said pulling operation.

3. The method of growing a single crystal of uniform resistivity, said method comprising pulling a single crystal from an initial melt of semiconductor material having a concentration of doping agent therein, causing a portion of a second melt of semiconductive material to be added to said initial melt at a rate dependent upon the rate at which semiconductive material is removed from said initial melt by said crystal-pulling step whereby the volume of said initial melt is maintained substantially constant during said pulling operation, and maintaining the concentration of said doping agent substantially constant during said pulling operation by adding substantially the same concentration of doping agent to said initial melt as is removed by said crystal during growth.

4. The `method of growing a semiconductor crystal of substantially uniform resistivity, said method comprising pulling a Crystal from an initial melt of semconductive material containing a doping agent, causing a portion of a second melt of semiconductive material to be added to said initial melt at a rate dependent upon the rate at which the rate of semiconductve material is removed from said initial melt by said crystal-pulling step Whereby the Volume of said initial melt is maintained substantially constant during said pulling operation.

5. The method of producng a crystal of substantially uniform resistvity, said method comprisng forming a crystal from an initial melt of semiconductve material containing a doping agent by causing a portion of said initial melt to progressively freeze into a solid state, causing a portion of a second melt of semiconductve material to be added to said initial melt at a rate dependent upon the rate at which semiconductve material is removed from said initial melt by said freezing step Whereby the Volume of said initial melt is maintained substantially constant as said Crystal freezes.

6. The method of growing a semiconductor crystal of substantially uniform resistivity, said method comprising pulling a crystal from an initial melt of semiconductive material having a predetermned concentration of doping agent therein, causing a portion of a second melt of said material to be added to said initial melt during said pulling operation at a rate dependent upon the rate at which said material is removed from said initial melt by said crystal-pulling step Whereby the Volume of said initial melt is maintained substantially constant during said pulling operation.

7. The method of groWing a Crystal of substantially uniform resistivity, said method comprising pulling a crystal from an initial melt of semiconductive material containing a doping agent, and adding additional semiconductive material to said initial melt as said crystal grows to replace that removed by said Crystal and maintain the Volume of said initial melt substantially constant during said pulling step.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,356 Sparks Mar. 17, 1953 2,727,839 Sparks Dec. 20, 1955 2,730,470 Shockley Jan. 10, 1956 2,768,914 Buehler Oct. 30, 1956 2,892,739 Rusler June 30, 1959 

1. THE METHOD OF GROWING A SINGLE CRYSTAL OF UNIFORM RESISTIVITY, SAID METHOD COMPRISING PULLING A SINGLE CRYSTAL FROM AN INITIAL MELT OF SEMICONDUCTIVE MATERIAL CONTAINING A DOPING AGENT, CAUSING A PORTION OF A SECOND MELT OF SEMICONDUCTIVE MATERIAL TO BE ADDED TO SAID INITIAL MELT AT A RATE DEPENDENT UPON THE RATE AT WHICH SEMICONDUCTIVE MATERIAL IS REMOVED FROM SAID INITIAL MELT BY SAID CRYSTAL-PULLING STEP WHEREBY THE VOLUME OF SAID INITIAL MELT IS MAINTAINED SUBSTANTIALLY CONSTANT DURING THE PULLING OPERATION AND ADDING A CONCENTRATION OF DOPING AGENT TO SAID INITIAL MELT SUBSTANTIALLY EQUAL TO THE CONCENTRATION OF DOPING AGENT REMOVED BY SAID GROWING CRYSTAL. 